The invention relates to an electronic control unit in particular an ignition control unit, comprising an electronic circuit in the form of a hybrid circuit arranged on a heat sink, and a plastic housing element containing the plug terminal jack, with a connection existing between the ground terminal of the hybrid circuit and a plug terminal of the jack.
Many electronic control units, for example ignition circuits in thick-film technology, consist of an electronic circuit, a heat sink and a housing with plastic frame. The electronic circuit usually contains one or more hybrid circuits mounted on the upper side of a ceramic substrate which electrically insulates it from the heat sink.
The plastic housing, in which a metal lead frame is integrated, is riveted to the heat sink, the rivet having to be inserted separately when assembling the housing of the ignition control unit. The part of the lead frame inside the plastic housing is connected by bonded wires to the electronic circuit and the outside part of the lead frame is part of the connector. One pin of the lead frame is used to connect the ground of the hybrid circuit to the ground pin of the connector.
In addition to the hybrid circuit/connector ground connection, most electronic control units have a wire bonded connection from the hybrid circuit to the metallic heat sink. Therefore the electronic ground of the hybrid circuit is connected with a very low ohmic resistance to the heat sink.
The electrical connection between the ground of the electronic circuit and the heat sink increases the so-called electro-magnetic compatibility of the control unit, i.e. it improves the behaviour of the control unit under the influence of electro-magnetic fields.
This additional ground connection must--in the event of a fault, i.e. when the ground pin of the connector breaks or becomes high--resistance--drain off the working current of the control unit, for example 7.5A in the case of ignition control units, via the ground bond/heat sink/mounting plate route, and so be sufficiently strong for this current. In addition, the hybrid layout must be designed so that the altered current path does not lead to ground coupling in the electronic circuit and an impairment of the functioning of the control unit. Furthermore, the additional ground connection must start from the single point ground of the circuit, so that no malfunctions of the control unit occurs if the ground pin of the connector is faulty.
These demands on the ground/bond connection can very often only be met with great difficulty, and result in higher space requirements both on the hybrid circuit and on the heat sink, as well as additional intersections on the hybrid circuit.
When assembling the electronic control unit, an additional bonding process from the hybrid circuit to the heat sink is necessary. To achieve a good contact of the bond to the heat sink, the heat sink has to be machined to get access to the bare metal.